When light frames are stacked, the signal to noise ratio (SNR) goes up as the noise drops. The more lights you use, the more the SNR increases; the stars have smoother profiles and the level of the noise drops and is smoother.

Figure 1 shows an X-Y-intensity plot of a star (HD 121228, V=7.83) taken from a single RAW image (left panel) and from a stack of 17 RAWs (right panel), nicely illustrating this effect.

Figure 2 graphs the increase in SNR of two stars as additional frames are added to the first light. For HD 121228 and the equipment used (details here) the SNR increased from 29 to 95. A brighter star (HD 120042, V=7.32) was also measured, and it's SNR went from 45 to a quite respectable 180.

Figure 2. Signal to noise ratio as a function of the number of light frames stacked, for HD 120042 (top curve) and HD 121228 (lower curve).